1. Field of the Invention
The present invention relates to a reactive power compensator device. More particularly, the present invention relates to the reactive power compensator device employing an AC capacitor and a power converter serially connected thereto, thereby compensating for reactive power to a distribution power system.
2. Description of the Related Art
Conventionally, most of loads in the distribution power system have the characteristic of inductance, and it will result in the poor power factor. Hence, it requires a larger current for transmitting the identical real power that reduces the power efficiency of distribution power system and degrades the performance of voltage regulation of the load side. For solving the above problems, power substations and power consumers generally install a passive type reactive power compensator (AC capacitors) parallel connected to the distribution power system, so as to compensate a lagging reactive power to increase the entire power factor. In some distribution power systems, the capacity of applied AC capacitor is about 25% to 35% of total capacity, and in some other distribution power systems have even exceeded about 50%, according to research reports.
However, the AC capacitors directly connect to the distribution power system in parallel have some disadvantageous. The AC capacitor merely provides with the fixed reactive power compensation that cannot be adjusted in response to the load variation that may cause high-voltage due to over-compensating the reactive power in the condition of light load. In consequence, high-voltage caused by the over-compensating reactive power may damage the other power facilities.
Recently, the harmonic pollution in the distribution power system becomes seriously due to the wide use of nonlinear loads. In the distribution power system, the AC capacitor used for power factor correction provides with a low impedance path for harmonic current, hence the AC capacitor is frequently damaged by harmonics. Meanwhile, it results in the power resonance between the AC capacitor and the reactance of the distribution power system. Then, it will result in the amplification of harmonic current and harmonic voltage. Thus, the damage of the AC capacitor due to over-voltage or over-current may occur. Besides, the over-voltage of AC capacitor caused by the power resonance may damage the neighboring electric power facilities and even result in public accidents. Consequently, it may affect the power quality and reliability for power supply.
In order to solve the power resonance problem caused by the AC capacitor, the voltage rating of AC capacitor is increased to thereby avoid the damage of over-voltage according to the conventional solution. However, it cannot eliminate the power resonance problem thoroughly and still, inevitably, remains the damage of neighboring power facilities.
There is another solution that the AC capacitor is switched off from the power system when over-voltage or over-current of the AC capacitor occurs. Hence, the function of reactive power compensation will be disabled.
To properly adjust reactive power provided by the AC capacitor and avoid the over-compensating reactive power in the condition of light load, a thyristor-controlled AC capacitor is disclosed in many patents, U.S. Pat. Nos. 5,969,509 and 6,462,519 for example. The thyristor switch is employed to control the capacitance of the AC capacitor for injecting reactive power into the distribution power system. However, the reactive power compensation is only adjusted step by step and cannot be adjusted in linear. Furthermore, U.S. Pat. No. 5,672,956 discloses a thyristor switch serially connected to a reactor that further connects to such an AC capacitor in parallel. In operation, the reactive power compensation can be adjusted in linear by controlling a firing angle of the thyristor switch. However, it may generate harmonic problem in the distribution power system.
The two above-mentioned solutions still employ the AC capacitor to connect to the distribution power system and cannot effectively eliminate the problems of power resonance and the injection of neighboring harmonic current that may damage the neighboring electric power facilities.
In order to solve the application problem of the AC capacitor for reactive power compensation, U.S. Pat. Nos. 5,187,427 and 5,329,221 disclose a power converter based reactive power compensator, as shown in FIG. 1. The power converter connects to the distribution power system via a filtering inductor. A DC side of the power converter further connects with a DC capacitor. Accordingly, the power converter can provide with either of a leading reactive power or a lagging reactive power by means of controlling the operation of the power converter. This means that the reactive power can be adjusted in linear. Advantageously, no power resonance problem occurs between the power converter and the distribution power system, and no problem of harmonic current injection due to neighboring nonlinear load. However, the power converter is employed to provide the entire reactive power compensation. Hence, the capacity of the power converter must be larger to provide the entire reactive power compensation in response to the full range load variation. Consequently, it may increase technical difficulty and manufacturing cost that limits the wide application of the power converter. Furthermore, U.S. Pat. No. 5,642,275 discloses a plurality of stacked power converters for large capacity reactive power compensation. However the manufacturing cost is still high.
The present invention intends to provide a reactive power compensator device employing an AC capacitor and a power converter serially connected thereto that can eliminate the problems of power resonance and the injection of neighboring harmonic current. The reactive power compensator device employs lower capacity and lesser number of power electronic switches of the power converter in such a way to mitigate and overcome the above technical problems of U.S. Pat. Nos. 5,187,427, 5,329,221 and 5,642,275.